1. Field of the Invention
The present invention relates to water distribution systems for evaporative coolers and, more particularly, to a water distribution system for controlling distribution of water uniformly across a media to avoid dry spots, scaling, streaking and distribution of excess water.
2. Description of Related Art
Evaporative cooling appears to be a simple process of passing hot dry air through a wet pad or media to evaporate the water with the latent heat of the air and inherently the air becomes cooler and more humid. In reality, there are three complex mechanical and chemical processes taking place in an evaporative cooler. The first process is the air system which is controlled by the pyschrometric chart and the efficiency of the media. The second process is the water delivery system that has to ensure that the media has sufficient water for evaporation and that the media is uniformly wetted. The third process is the water chemistry system where the water for evaporation is controlled so that the naturally occurring dissolved solids in the water remain in solution and are disposed of prior to being deposited on the media. Almost all evaporative coolers built to date have made only first order approximations for one or more of the processes and have either ignored or been unaware of the others.
The air around us is essentially a constant composition of gases (nitrogen, oxygen, carbon dioxide and others) and varying amounts of water vapor. It also contains solid impurities such as dust and organic material, which will be ignored in the following discussion. The gas component of air behaves in accordance with Boyle's and Charles' laws, i.e. the volume of the gas varies inversely with the absolute pressure and directly with the absolute temperature, respectively. The water vapor portion of air does not behave as a perfect gas. The amount of moisture in the air is dependant on the amount of moisture available and is limited to a maximum saturation value based on the air temperature and pressure. As moisture is added to or removed from the air, water is either evaporated or condensed. This change in phase captures or releases energy. In evaporative cooling applications, the evaporation of water absorbs heat. The movement of the heat from the air to the water vapor happens without a change in air volume or air pressure and results in a lowering of the temperature of the air. The relationships between pressure, temperature, humidity, density and heat content are most commonly shown graphically on pyschrometric charts. These relationships are very well defined and have been the subject of extensive research. Applying the pyschrometric chart to the evaporative cooling process is easy for any one particular set of operating conditions. If one knows the entering air temperature (inlet dry bulb), the relative humidity of the inlet air, the barometric pressure and the volume of air being cooled one can calculate the theoretical amount of moisture that can be evaporated into the airstream and the resulting temperature reduction.
Actual operating conditions change constantly. The inlet air temperature, the relative humidity and barometric pressure are the detailed measurements of what is generally referred to as the “Weather”. Most evaporative cooler manufacturers design their equipment to handle a specific air flow rate at standard conditions and size the evaporation media for this flow rate. The efficiency of the evaporative cooler is determined by the air flow rate over the chosen media. Each media type has physical characteristics that determine how fast and thoroughly the water can be evaporated into the airstream. The most common evaporative cooling media in use today is a corrugated kraft type paper. The market leader in this type of media is Munters Corp. which markets its media under the brand names Cel Dek and Glacier-Cor. Depending on the thickness of the media used and the velocity of the air flowing through the media, the saturation effectiveness (efficiency) can range from less than 60% to about 98 or 99%.
The majority of existing evaporative coolers are controlled by a downstream thermostat and the evaporative coolers are either on or off. The efficiency of the evaporative cooler changes with the weather and the water system pressure. The conventional evaporative cooler does not attempt to control any of these process variables to optimize efficiency.
To obtain maximum evaporation, the media must be adequately wetted. Most conventional evaporative coolers have a large basin or sump filled with water that is pumped to a perforated header pipe at the top of the media. The water is sprayed from the header pipe up to a deflector shield and runs down onto the top of the media. Excess water is applied to ensure saturation of the media. The water not evaporated drains into the sump to be reused. All recirculating evaporative cooler manufacturers recommend that a portion of the recirculating water be discarded and replaced with fresh water added to the sump to keep the water quality at a minimum quality level.
The media removes significant amounts of airborne contaminants from the air as it passes through the media and the return water rinses a portion of the contaminants off the media and carries them to the sump. In addition, naturally occurring salts in the water supply become concentrated on the surface of the media and are partially rinsed into the sump. While some of these contaminants and precipitated salts settle to the bottom of the sump, a significant amount are entrained at the pump inlet and are recirculated back onto the media.
The pumps used in most recirculating type evaporative coolers are submersible centrifugal pumps. These inexpensive pumps are not precision pieces of equipment when new and wear quickly as the debris is recirculated. This deterioration of the pump leads to fairly rapid changes in the delivery head for the pump. This change in the output of the pump renders it difficult to regulate the water flow across the media. The distribution header pipe uses large holes on relatively large hole spacing to minimize debris fouling and plugging. The end result is an uneven water distribution and occasionally dry strips on the media. Constant maintenance is required to adjust and maintain an adequate supply of water for the media. Often, these systems attempt to cure uneven water flow by pumping an excess amount of water to the media. This excess amount of water can cause the cellulose media to deteriorate prematurely with associated poor performance and costly early media replacement.
The most overlooked aspect of evaporative cooling is controlling the concentration of dissolved solids in the water being evaporated on the media. The water supply for evaporative coolers is typically domestic water which contains a number of compounds as dissolved solids. Water is evaporated by the warm air leaving behind all of the dissolved solids in a small volume of water on the media. Each type of dissolved solids has a solubility limit. That is, when the concentration of a particular compound reaches a known concentration, the compound precipitates out. In evaporative coolers the most common form of precipitate is calcium carbonate scale on the media. This hard water scale does not re-dissolve when rewetted. Once formed on the media it reduces the saturation efficiency and clogs the water distribution channels.
Recirculating evaporative coolers reapply the sump water to the media. Each time the water is applied, some of it evaporates and the dissolved solids build up in the water. All evaporative cooler manufacturers either bleed some of the recirculating water off to try and reduce the concentration of the dissolved solids (called cycles of concentration in the industry) or dump the sump water occasionally to eliminate as much of the dissolved solids as possible. Most sumps have a float actuated make up valve to add water to the sump. This mixes the fresh water with the concentrated dissolved solids in the water and reduces the concentration. As a practical matter, however, the resulting water being distributed on the media will always have higher levels of dissolved solids than the inlet water.
If the water distribution system allows the water in any area to become too concentrated with dissolved solids before it leaves the media, the media will start to scale. Once scaling begins, the process threshold for additional scaling is reduced such that the salt crystals will grow whenever the water surrounding them is just near the precipitation point.
Effectively controlling the build up of scale through deployment of a feed and bleed process requires more control equipment and is currently supplied with such systems. To date, a clear solution is that of eliminating a recirculating system in favor of a single water pass system. The single pass systems provide water to the top of the media and let it flow through the media and the flow therefrom is drained. Several challenges must be overcome to implement such a system. First, one must incorporate on/off controls to regulate the water introduced to the media. Second, the flow volume of water must be sufficient to wet the media completely and yet the flow must be periodically shut off to avoid wasting large amounts of water. Some existing systems use a timer based controller to regulate the water flow. Another type of system uses a single temperature sensor within the media coupled with a timer to control the flow of water. These systems typically fail prematurely either from using too much water or from using an insufficient amount of water resulting in drying out and scaling of the media. Neither of these two types of systems are widely commercially acceptable.
In general, the evaporative cooler market has become a commodity market, with market conditions forcing the manufacturers to produce less expensive coolers. Without clear standards on how to rate the units and a consumer base untrained in the art of evaporative cooling will not recognize the consequences of the current industry practice to rate evaporative coolers at a nominal air flow rate without reference to the efficiency of the unit. As a result, the consumer makes his decision primarily on cost rather than performance or return on investment.
Various prior art evaporative cooler systems are described in the patents listed below.
U.S. Pat. No. 4,968,457 describes a non circulating control for an evaporative cooler. The water flow is metered by a simple solenoid valve which does not take into consideration change in flow rate as a function of inlet line pressure. Therefore, the amount of water delivered at different times of the day will vary with changes in domestic water line pressure. Furthermore, there is no understanding of the need for a change of water flow rates as a function of the hardness of the inlet water nor is there a discussion of providing more water than is evaporated to keep the media from scaling. A sensor for controlling operation of a solenoid valve is placed downstream of spray nozzles ejecting water to the media to sense the temperature or the humidity. There is no understanding that the cooling process is primarily dependant on the inlet air conditions.
U.S. Pat. No. 5,775,580 is directed to a non circulating evaporative cooler for primarily eliminating the dripping of water from the media. This will result in at least a part of the media becoming dry with resulting deposit of salts and compromise of the integrity of the media and its effectiveness unless pure water is used.
U.S. Pat. No. 6,367,277 discloses the use of fresh water makeup to minimize scaling in a recirculating evaporative cooler system. There is no disclosure relating to controlling the hardness of the water at the point of evaporation on the media nor does this system minimize the amount of water used. It also requires bleed of a substantial amount of the recirculating water to keep the minerals from precipitating out. No understanding of the varying conditions from location to location and the effect thereof on the efficiency of the evaporative cooler is set forth.
There are several types of problems associated with heavy scale formation on the media in an evaporative cooler where evaporative cooling occurs. First, there is a decreased air flow through the media because the air channels therewithin become more or less plugged. To maintain an adequate air volume, the velocity of the air through the media must increase. At speeds above 650 feet per minute, there is a tendency for small droplets of water to become entrained in the airstream unless other steps are taken. These droplets may super saturate the airstream to the point that moisture may condense downstream of the media and create other problems unacceptable to the user. Second, at localized concentrations of salts, the pH in those areas increases dramatically. The high pH will allow the water to leach the resin and delignify the cellulose in the media and cause premature structural failure of the media.
Indoor air quality has become a growing concern as modem office and industrial buildings become more energy efficient and better insulated. Various regulations cover how much fresh outside air must be introduced into the HVAC system in a building. This outside air is rarely at the desired temperature and relative humidity. In the southwest of the United States, the air is generally much dryer and hotter then desired. This means that the makeup air requires cooling and humidification before it can be introduced into the building. Conventional chilled water systems in large commercial buildings use a combination of cooling towers and mechanical chillers to supply the cooling for the building. These systems use considerable electricity to operate. Direct evaporative cooling has been used to reduce this electrical demand by cooling the makeup air during its introduction into the building. These applications have been plagued by the same scaling and lack of control problems described above.
Evaporative cooling is often used in dusty industrial environments. Historically, recirculating evaporative coolers become plugged with dust. Often pre-filters are installed upstream of the evaporative cooler to remove the dust present in the air. Poor maintenance often resulted in filter overloading, filter failure and media plugging. One approach to this problem of dust has been that of using an excess water flow controlled by only a timer for dust control. These results were not particularly successful. A further unit uses a fresh water makeup header to try to control the dust buildup with a timer to activate the flush. This has not proven to be particularly effective.